By law, voting day is set for Oct. 21, but the prime minister has a two-week window, beginning on Sept. 1 in which to begin the formal campaign.

The federal parties have been campaigning informally for weeks, with Liberal ministers making government funding announcements while Conservative Leader Andrew Scheer and NDP Leader Jagmeet Singh have attacked the Liberal government's record.

They have argued over climate-change policy, Canada's place in the world, federal spending and whether a national economy that has performed well by statistical measures has left Canadians feeling financially secure.

Recent opinion polls have suggested the Liberals and Conservatives are fighting for first place in popular support, but neither would likely win a majority in the House of Commons if an election were held immediately.